There are rules governing the content of these insults and their use in discourse. In terms of content, the insults must be about someone else (typically the mother), not the person the insult is addressed to. The insult typically refers to appearance, age, poverty, or sexual promiscuity, and, most important, the insult must not literally be true. Guaranteeing the falsity of insults depends in part on shared knowledge. For example, the insult that your mother is a prostitute would not be addressed to a boy whose mother really was a prostitute. Falsity is also ensured by making the insults so extreme as to be literally impossible.